1. Field of the Invention
This invention relates generally to electrical signal filters and more particularly to an active filter for controlling ripple in the load current or source current of DC power systems.
2. Description of Related Art
Various systems have been developed for controlling ripple, broadly defined as xe2x80x9cany unwanted time-varying voltage or currentxe2x80x9d, in DC power systems. Such means include passive filters, voltage and current regulators, and active filters. Each of these approaches has its advantages and disadvantages.
Passive filters are perhaps the most widely used means of controlling ripple. Such filters have low complexity and high efficiency. Passive filters, moreover, are better suited for attenuating high frequencies than low frequencies. At low frequencies, the size and weight of the passive components typically become prohibitive, particularly for mobile, airborne, and space borne systems.
Voltage and current regulators are better suited for attenuating low frequency ripple than high frequency ripple. This is due to the fact that they employ active control loops which have limited bandwidth. Voltage and current regulators may be of the linear type or the switching type. Linear regulators have lower complexity, but tend to have low efficiency. Switching regulators, on the other hand, have high efficiencies, but they are more complex and can introduce unwanted high frequency ripple while attenuating low frequency ripple. The regulation provided by voltage and current regulators is an added benefit not offered by passive or active filters.
Active filters, like voltage and current regulators, operate better at low frequencies than at high frequencies and may also be of the linear type or switching type. Active filters cancel ripple signals by injecting voltages or currents that are equal to the unwanted signals in amplitude but opposite in phase. While electronic regulators must process all of the power required by load, active filters process only the power associated with ripple that is being canceled. Consequently, active filters can utilize smaller components with lower power ratings than their regulating counterparts. In systems where regulation is not required or is provided by other means, active filters may have an advantage over voltage and current regulators in terms of cost, size, weight, and efficiency.
Accordingly, it is an object of the present invention to provide an improvement in electrical filter circuitry.
It is another object of the invention to provide an improvement in filter circuits for controlling ripple in DC power systems.
It is a further object of the invention to provide an improvement in active filters for controlling ripple in DC power systems.
These and other objects are achieved by a circuit for filtering unwanted AC voltages and currents from a DC power circuit, and is comprised of a power circuit and a control circuit. The power circuit consists of two semiconductor power switches and respective anti parallel diodes, an inductor and at least one capacitor. The control circuit consists of a multi-loop feedback network that produces a duty cycle command signal and converts it to a pair of complementary pulse width modulated (PWM) drive signal for the two semiconductor power switches. The filter, operating on the principle of a bi-directional boost/buck converter, implements an auxiliary DC voltage bus which by charging the capacitor acts as a reservoir of energy. The DC level of this auxiliary bus is controlled by controlling the DC component of the duty cycle. The AC component of the duty cycle is used to modulate the current drawn from or supplied to the power system with the point of injection located between the DC source and the load. The modulated current is controlled in such a way as to cancel unwanted currents or voltages within the power system.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while indicating the preferred embodiment of the invention, is provided by way of illustration only, since various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.